Tissue-air ratio and its associated 'in-air' reference minimal phantom

A problem is raised concerning a revised equation published in 1981 relating tissue-air ratio to per cent depth dose for an incident photon beam. The problem mainly involves the required size of the reference minimal phantom that is irradiated free-in-air to yield the reference absorbed dose as a tissue-air ratio. If the radius of the minimal phantom is significantly less than the range of the most energetic secondary electrons, the reference absorbed dose is indeterminate. It is shown that the small phantom size assumed in the 1981 publication could result, even if no electrons contaminate the incident photon beam, in a large discrepancy. (U.K.)